| dc.description.abstract | Droplets, in their myriad forms and functions, have become indispensable players in the world of microreactors, drug delivery systems, crystallization studies, and cell culture platforms. These tiny, self-contained units offer tremendous potential but also pose significant challenges, particularly concerning droplet encapsulation, contamination, mechanical stability, substrate dependence and evaporation resistance. Liquid marble (LM), a hydrophobic particle coated droplet, can offer solutions to some of these problems. The hydrophobic nature of the particles ensures their retention at the droplet interface, forming a protective layer around it. Despite this protective layer, mechanical stability and rapid evaporation of such droplets are still a major challenge, hindering many practical applications.
To unravel the intricacies associated with mechanical robustness, it is fundamentally important to understand how would particle at the interface behave under various conditions such as extreme deformation and vibrations. Thus, in the first part of the research, we studied the impact deformation of LM on non-wetting surfaces. An intriguing phenomenon surfaces, revealing an interfacial fingering instability that suppresses pinch-off in these particle-coated droplets, giving rise to mechanical stability even at higher impact forces. This phenomenon, observed at significantly lower impact energies than bare droplets, is characterized and explained using the rim Bond number. The onset of the instability leads to the formation of stable fingers, contributing to higher losses that suppress pinch-off. The implications extend beyond droplets, as similar instabilities can be observed in dust/pollen-covered surfaces, rendering the findings applicable to cooling, self-cleaning, and anti-icing applications. The investigation then shifts to the critical realm of contact-time reduction during droplet impact, crucial for applications like self-cleaning, anti-icing, and heat transfer. The introduction of micro-nano hydrophobic particles on droplet surfaces presents two distinct modes of contact-time reduction. At lower impact energies, reduced adhesion between particle and surface contributes to a reduction of up to 21%, independent of particle size but dependent on the solid fraction of LM. For larger particle sizes and higher impact energies, a fragmentation-based reduction of up to 65% is observed, where the spreading LM lamella breaks due to its thickness becoming similar to particle dimensions. These findings underscore the potential of LM as a novel approach for contact-time reduction during droplet impact, offering applications in various scientific and industrial fields.
Moving beyond impact and deformation dynamics, the thesis addresses challenges in droplet encapsulation, particularly in tuning micron-scale shell thickness over a wide range of droplet sizes. Traditional methods such as microfluidics and dip coating have struggled to provide a one-size-fits-all solution for encapsulating droplets of varying sizes. We introduced a novel capillary force-assisted cloaking technique using hydrophobic nano-microparticles and liquid-infused surfaces. In this innovative approach, the configuration, termed Liquid Marble on Oil-infused surfaces (LMOI), combines the unique properties of liquid marbles with oil-infused surfaces to achieve remarkable tunability in shell thickness and enhanced droplet stability. This technique enables uniform solid and liquid shell encapsulations with tunability in thickness (5-200 μm) for a droplet volumes spanning over 4 orders of magnitudes. The tunable liquid encapsulation demonstrates a remarkable reduction in evaporation rates, up to 200 times, with wide tunability in lifetime (1.5 hrs to 12 days). The potential of capillary force-assisted cloaking goes far beyond droplet encapsulation alone. We have demonstrated its utility in single crystal growth for a diverse range of substances, including copper sulfate, Rochelle salt, sodium nitrate, and Lysozyme protein. The feasibility of employing this method for biological applications has been tested successfully, with human and yeast cells thriving in a hanging droplet configuration. Solid capsules, designed to respond to external stimuli such as temperature changes, offer yet another dimension of tunability. Finally, Merging dynamics of LMOI with bare droplets and other LMOI are investigated. In contrast to merging of bare droplets, coalescence involving at least one LMOI reveals a three-step process, including spreading, depletion, and eventual merging phases. Higher oil viscosity influences the merging process, with increased viscosity leading to delayed merging involving droplet with longer spreading and depletion phases. LMOI exhibits significant resistance to merging with another LMOI, necessitating external triggers like pressure or electric fields for coalescence. These findings provide insights into designing microreactor systems based on LMOI, contributing to the comprehension of their dynamics and functionalities.
In summary, this thesis explores the diverse applications of droplets, emphasizing the transformative potential of liquid marbles (LM) and novel encapsulation techniques. From impact dynamics to contact-time reduction and tunable shell thickness, the research unveils innovative approaches for enhancing droplet stability and functionality, offering valuable insights for microreactors, drug delivery, and cell culture platforms. | en_US |
